Conventionally, as a brake mechanism for a vehicle, there has been known a booster, for example, with use of a hydraulic booster or a negative pressure booster. In recent years, as a booster of this type, there has been disclosed an electric booster with use of an electric motor as a boosting source (refer to, for example, Patent Document 1).
The electric booster disclosed in Patent Document 1 is configured as an integrated single device including a main piston (input piston) which is reciprocated by an operation of a brake pedal, a cylindrical booster piston which is fitted onto the main piston so as to be displaceable relative to the main piston, and an electric motor which reciprocates the booster piston.
In this case, the electric booster has the main piston and the booster piston as pistons of a master cylinder and has each of front end portions of the pistons facing each pressure chamber of the master cylinder, and generates a brake hydraulic pressure in the master cylinder, by an input thrust force applied to the main piston from the brake pedal and by a booster thrust force applied to the booster piston from the electric motor.
Incidentally, in the electric booster disclosed in Patent Document 1, for example, when a hydraulic pressure is applied from an outside to the pressure chamber which is faced by the main piston of the master cylinder, the main piston moves backward to the booster piston side by an operation of the hydraulic pressure, but it becomes difficult to ensure a minimum hydraulic pressure in the master cylinder if the main piston moves backward too much. In this case, a backward movement of the main piston can be, for example, restricted by a restriction unit or the like, but there is a possibility that a piston sliding surface of the cylinder body is deformed by a load which is applied to the restriction unit and transmitted to the cylinder body.
Further, in the electric booster disclosed in Patent Document 1, for example, when a pressure is applied from the outside to the pressure chamber which is faced by the main piston of the master cylinder, a restriction unit which restricts a backward position of the main piston is required in order to ensure a minimum brake hydraulic pressure in the master cylinder. However, in this case, there is a concern that an excessive force is applied to the restriction unit.
Meanwhile, there has been known a vehicle brake device including a master cylinder into which a brake operation of an operator is inputted, and a slave cylinder as an electric brake device which generates the brake hydraulic pressure based on an electrical signal in accordance with the brake operation (for example, see Patent Document 2).
In the slave cylinder of the vehicle brake device, when the motor is driven, the piston moves forward via a gear mechanism, and a ball screw mechanism having a nut and a screw which is engaged with the nut. That is, the nut receives a rotational driving force of the motor, and the screw engaged with the nut moves in the axial direction to press the piston, and thus the piston is moved forward. Thus, the brake hydraulic pressure is generated in a hydraulic chamber which is formed in front of the piston, and the brake hydraulic pressure is outputted to the fluid passage through a port.
However, in the slave cylinder, the screw moves in the axial direction by a rotation of the nut during a forward movement of the piston, and a decentering (shake; deviation from the reference center position of the axis) of the screw tip increases in response to an increase of a projection amount of the screw from the nut, and thus there is a possibility that an inclination of the piston pressed by the screw occurs. Such inclination of the piston leads to a damage or abrasion of the piston, a seal member attached to the piston, or the like.